Peptidoglycan (PGN) is a highly-conserved pathogen-associated molecular pattern that is present in especially high amounts in Gram-positive bacteria. Humans and non-human primates respond to PGN in vitro by production of inflammatory cytokines and platelet activation and in vivo by systemic inflammation and disseminated intravascular coagulation (DIC). Paradoxically, mice and mouse cells show minimal responses to PGN. We recently discovered that both humans and non-human primates naturally express IgG antibodies that cross-react with PGN derived from a variety of pathogens, while mice lack these antibodies. The anti-PGN antibodies were essential for PGN responses, based on the following observations: 1. All surveyed healthy humans express anti-PGN antibodies that opsonize PGN. 2. PGN activates the classical complement pathway in human serum and plasma. 3. Intact but not F(ab')2 fragments of normal human IgG support PGN-triggered inflammatory cytokine production in human innate immune cells. 4. Expression of the human IgG receptor Fc?RIIa in HEK293 cells permitted the cells to bind and phagocytose PGN in a human IgG-dependent manner. 5. Baboons responded to an in vivo challenge of PGN with features of systemic inflammation and DIC; the response directly correlated with their individual titer of anti-PGN IgG antibodies. We hypothesize that the presence and level of anti-PGN IgG is associated with the pathology caused by PGN and thus of Gram-positive pathogens. This project will directly test whether the presence of the anti-PGN IgG in humans contributes to the pathogenic effects of PGN by passive transfer of human IgG containing anti-PGN antibodies to human Fc?RIIA-transgenic mice. We will also generate a mouse model for PGN-induced pathology by making mice immune to PGN, challenging immune or naive mice in vivo, and measuring signs of inflammation. A mouse model will be an important breakthrough as it will permit a better understanding of the response to the PGN pathogen-associated molecular pattern, shared by all Gram-positive pathogens.